1. Field of the Invention
This invention relates to an image forming apparatus such as a copier or a printer utilizing the electrophotographic process or the like, and particularly to an apparatus for effecting the density control and gradation control of an image formed thereby.
2. Related Background Art
With the development of the stream of an information-oriented trend, needs for handling documents and images in colors are spreading and printers of various types are on the market. As processes for forming color images, use have been made of the sublimation type, the heat transfer type, the ink jet type, etc., and to form images at a high speed, the electrophotographic type is said to be most excellent.
In an image forming apparatus of the electrophotographic type, there is the problem that image density is greatly fluctuated by the temperature and humidity under which the apparatus is used, and the unevenness or the like of the characteristics of a photosensitive member and developers. Particularly regarding color images, there arises the inconvenience that color taste is changed.
In view of these problems, in a color image forming apparatus, there is generally carried out density control in which a patch (pattern) which is an image for density control is formed in advance on a photosensitive body, an intermediate transferring member, a transferring material conveying member, a transferring material or the like, and the density thereof is detected by a density detecting sensor to thereby control image forming process conditions such as a charging bias, a developing bias and an exposure amount, and stabilize image density.
Also, in an image forming apparatus of the electrophotographic type which outputs a gradation image, the relation between an inputted image signal and the density of the output image, i.e., the gradation characteristic, generally has no linear relation, and on the low density side, density is low relative to the image signal and conversely, on the high density side, density is high relative to the image signal.
With this gradation characteristic kept intact, it is usually impossible to obtain images of a high quality of image and therefore, there is generally carried out gradation control (halftone control) in which a patch (pattern) which is an image for gradation control is tentatively formed on a photosensitive drum by a predetermined image signal, and the density of the patch is detected by a density sensor or the like, and from the result of the detection, the gradation characteristic of the image forming apparatus at that point of time is found, and on the basis thereof, a look-up table (LUT) is prepared, and the gradation characteristic is adjusted by the LUT so as to assume an appropriate relation such as a linear relation.
As density sensors used during the density control and the gradation control, a common one is usually used from the viewpoints of manufacturing cost and mounting space.
The measuring location for the patch density may be on the photosensitive body, on the intermediate transferring member, on the transferring material conveying member or on the transferring material.
However, when the density sensor is provided on the photosensitive body, the density sensor may be stained by a toner and the degree of freedom of the disposition of the density sensor is small and therefore, it is preferable to dispose the density sensor on the intermediate transferring member, the transferring material conveying member or the transferring material.
Also, the detection timing by the density sensor is the same for the density control and the gradation control, and has usually been after patches of four colors have been formed.
However, the image forming apparatus in which the above-described density control and gradation control are carried out has suffered from the following problems. Description will hereinafter be made with a four-color full color image forming apparatus comprised of an intermediate transferring member and a photosensitive drum taken as an example.
In this apparatus, when a toner image of the first color on the intermediate transferring member contacts with the photosensitive drum, part of a toner constituting the toner image is retransferred from the intermediate transferring member to the photosensitive drum. As the result, the toner image of the first color is reduced in density before transferred to the transferring material, as compared with immediately after transferred to the intermediate transferring member. Toner images of the second color and the third color, although differing in degree, are also reduced in density before transferred to the transferring material, as compared with immediately after transferred to the intermediate transferring member.
Against the above-noted problem, it will be enough if design can be made such that the retransfer becomes low by the selection of a transfer bias, but a transfer bias (transfer voltage) with which low retransfer and high transfer are compatible varies with the temperature and humidity under which the image forming apparatus is used, the unevenness of the characters of the photosensitive body and the developers, the degree of use thereof, etc. It also depends greatly on toner density (toner amount).
FIG. 7 of the accompanying drawings is a graph in which the transfer efficiency and retransfer efficiency when the transfer bias was changed are plotted. In FIG. 7, solid lines indicate the transfer efficiency, and the ratio between the toner amount MIS per unit area on the photosensitive body and the toner amount M/S when the toner image was transferred onto the intermediate transferring member is indicated in %. Dotted lines indicate the retransfer efficiency, and the ratio between the toner amount M/S on the intermediate transferring member and the toner amount M/S on the photosensitive body after the photosensitive body was contacted is indicated in %. The graph means that the higher is the retransfer efficiency, the more toner on the intermediate transferring member is transferred to the photosensitive body side. Also, the mark "Ovalsolid" indicates a case where M/S on the photosensitive drum is as small as 0.4 mg/cm2, and the mark x indicates a case where M/S on the intermediate transferring member is as great as 0.8 mg/cm2.
As can be seen from FIG. 7, to satisfy high transfer efficiency at M/S=0.8 mg/cm2, the transfer bias must be made high and in that case, the retransfer efficiency is high and aggravated.
During ordinary printing (image formation), the maximum M/S is of the order of 0.6 mg/cm2 and therefore, use can be made of a low transfer bias which can lower the retransfer efficiency, but during the detection of the patch for density control, it is necessary to form up to a high-density patch and therefore, a transfer bias satisfy high transfer efficiency and low retransfer efficiency could not be selected.
As a method of solving the reduction in density by the retransfer of the patches for density control, the detection of the patches can be effected immediately after the transfer to thereby eliminate the influence of the retransfer, but in this case, if the detection of the patches for gradation control was effected immediately after the transfer, the following inconvenience occurred.
The patches for gradation control are formed for high toner density (toner amount to low toner density in order to grasp the gradation characteristic of the apparatus, and the toner density (toner amount) is detected, but the retransfer depends on the toner density (toner amount) and therefore, the gradation characteristic when there is the influence of the retransfer and the gradation characteristic when there is not the influence of the retransfer differ greatly from each other. To effect gradation control with good accuracy, it is necessary to detect the density of the patches for gradation control under a condition conforming to ordinary printing influenced by the retransfer or the like. Consequently, if the detection of the patches is effected immediately after the transfer to thereby eliminate the influence of the retransfer, a reduction in the accuracy with which the gradation control is effected will occur.
The foregoing description has been made with respect to a color image forming apparatus comprised of an intermediate transferring member and a photosensitive drum, and again in a color image forming apparatus in which, instead of using an intermediate transferring member, a transferring material is carried on a transferring material conveying member and conveyed to a photosensitive drum, a similar problem arises when a patch is formed on the transferring material conveying member.
Also, in recent years, because of the requirement of the market for still a higher speed of image forming apparatuses, color image forming apparatuses of the in-line type in which color forming units of four colors are juxtaposed relative to an intermediate transferring member or a transferring material conveying member have begun to be on the market. In the color image forming apparatus of this in-line type, when density control and gradation control are to be effected, it would occur to mind to provide a density sensor for the photosensitive member of each unit, but from the viewpoints of the manufacturing cost and mounting space, a density sensor is generally disposed for the intermediate transferring member or the transferring material conveying member, and again in this case, the reduction in density and the deterioration of the accuracy of gradation control by the retransfer of a patch for density control pose a problem.
This problem of retransfer, in an image forming apparatus of the in-line type wherein a cleaning apparatus for a photosensitive drum is eliminated to thereby realize a cleanerless system, results in the problem that a toner of other color is collected by a developing device due to retransfer and developers are mixed with each other in the developing device to thereby greatly deteriorate the quality of image.
It is an object of the present invention to provide an image forming apparatus which maintains the accuracy of gradation control by a patch for gradation control, and prevents any reduction in density by the retransfer of a patch for density control.
It is another object of the present invention to provide an image forming apparatus comprising an image bearing member, a moving member, transferring means for transferring an image formed on the image bearing member onto the moving member by applying a voltage thereto, and detecting means for detecting an image for density control and an image for gradation control on the moving member transferred from the image bearing member, wherein a density and gradation of the image formed on the image bearing member are controlled on the basis of a result of a detection by the detecting means, and wherein a voltage when the image for density control formed on the image bearing member is transferred onto the moving member and a voltage when the image for gradation control formed on the image bearing member is transferred onto the moving member differ from each other.
It is still another object of the present invention to provide an image forming apparatus comprising an image bearing member, a moving member, transferring means for transferring an image formed on the image bearing member onto the moving member in a transferring portion, and detecting means for detecting an image for density control and an image for gradation control on the moving member transferred from the image bearing member, wherein a density and gradation of the image formed on the image bearing member are controlled on the basis of a result of the detection by the detecting means, and wherein the image for density control is detected by the detecting means after the image for density control is transferred from the image bearing member onto the moving member and before the image for density control on the moving member passes through the transferring portion, and the image for gradation control is detected by the detecting means after the image for gradation control on the moving member transferred from the image bearing member passes through the transferring portion.
Further objects of the present invention will become apparent from the following description.